The present invention relates to fluidization additives to fine powders, and more particularly the present invention relates to fluidization additives to fine paint powders.
Fluidization occurs when particulate materials having sizes ranging from sub-micrometers to several millimeters are suspended by up-flowing gas in a vessel or column which results in a gas-solid suspension, more commonly referred to as a fluidized bed. The fluidized beds formed with the gas-solid suspension are specifically referred to as gas-solid fluidized beds. The term xe2x80x9cfluidized bedxe2x80x9d applies because the gas-solid suspension formed by the solid particles and the upflowing gas behaves like a fluid. A gas-solid fluidized bed can operate in several fluidization regimes: particulate, bubbling, slugging and turbulent fluidization regimes (collectively called conventional fluidized beds), and fast fluidization and pneumatic transport regimes (collectively called high-velocity fluidized beds). There is a minimum gas velocity, called minimum fluidization velocity, below which the bed is not fluidized.
Key characteristics of fluidized beds include easy handling of particles, excellent contact between gas and solids, excellent heat and mass transfer between gas and solids and between gas-solid suspensions and the column wall, and good mixing of gas and solids to mention a few. These and other useful characteristics have led to the wide application of fluidized beds in powder processing and related industries. The xe2x80x9ceasy handling of particlesxe2x80x9d in fluidized beds is due to the uniform solids suspension inside the bed and the relatively free movement of the particles within the gas-solids suspension and of the suspension itself.
Powders may be classified into four groups in gas-solid fluidized systems, according to Geldart""s classifications (Geldart, xe2x80x9cThe Effect of Particle Size and Size Distribution on the Behavior of Gas Fluidized Bedsxe2x80x9d, Powder Technology, Volume 6, 210, 1972 and Geldart, xe2x80x9cTypes of Gas Fluidizationxe2x80x9d, Powder Technology, Volume 7, 285, 1973). Groups B and D powders comprise large particles that typically result in large bubbles when fluidized. Group A powders comprise particles that first experience a significant expansion of the powder bed when fluidized before bubbles begin to appear. Both Group B and Group A powders can be well fluidized. Group C powders comprise very small (fine) particles for which interparticle forces significantly affect the fluidization behaviour in such a way that fluidization of these powders is very difficult. Typically, as the particle size reduces, interparticle forces increase significantly causing the fine particles to agglomerate since they become very cohesive. Typical Group C powders comprise particles under 25-35 xcexcm in average size, although powders larger than these sizes which are very cohesive may also belong to the Group C powders. Thus, due to strong interparticle forces, Group C powders are either very difficult to fluidize (with channeling and/or very poor fluidization) or they primarily fluidize with the large agglomerates as pseudo-particles rather than as individual particles. In either case, fluidization of individual particles cannot be achieved easily which makes handling of Group C powders problematic. Group C powders also tend to clog up in certain areas of the fluidized bed, powder transport lines and powder processing equipment, such as above the gas distributor and around internals and at exit port(s), and tend to stick to the internal wall, corners or the ceiling of the bed, transport lines and other powder processing equipment.
There are many processes or uses for which Group C fine powders need to be handled. To enhance their flowabilities, different measures have been taken to assist the fluidization and transportation of these Group C powders. Those methods are usually referred to as fluidization aids. Fluidization aids include mechanical stirring, acoustic, mechanical or ultrasonic vibration, addition of much larger particles to provide extra stirring, pulsation of fluidization gas, to mention just a few. Some of these measures are more effective than others for a given Group C powder, but the effectiveness of almost all of these measures tends to diminish as the powder becomes finer or smaller in size. As used herein, the terms xe2x80x9cfluidization aidsxe2x80x9d, xe2x80x9cflow aidsxe2x80x9d and xe2x80x9ctransportation aidsxe2x80x9d are referring to the additional measures or methods applied to the fluidized bed and/or powder to enhance the fluidization and handlability of fine powders, while the terms xe2x80x9cfluidizabilityxe2x80x9d, xe2x80x9cflowabilityxe2x80x9d, xe2x80x9chandlabilityxe2x80x9d and xe2x80x9ctransportabilityxe2x80x9d are referring to the same general concept, that is, the ability of a powder to flow better and therefore to be handled and transported more easily.
Another method of increasing fluidizability of powders involves the addition of some silica or alumina finer particles (additives). For example, it has been known that adding a small fraction of extremely fine silica powder improves the fluidization of Group C powder. On the other hand, addition of many other finer particles has been observed not to help in the fluidization of fine powders. Therefore, the mechanism is not yet clearly understood, although some have speculated a xe2x80x9clubricantxe2x80x9d effect. As used herein, the terms xe2x80x9clubricantxe2x80x9d, xe2x80x9clubricating agentxe2x80x9d and xe2x80x9cadditivexe2x80x9d are all referring to solid additives that are added to the finer powder, aiming at enhancing its fluidization.
An example where it is extremely important to maintain good fluidization and transportation of fine powders is powder coating. Powder coating is a process superior to the traditional liquid coating process. A traditional paint application technique, referred to as xe2x80x9cwet coatingxe2x80x9d, involves the application of a liquid paint where solid paint components are first dissolved into or suspended in a solvent which is then applied to the surface of the part being painted. Polymerization and/or other reactions of the paint components then occurs in the wet paint layer on the surface, leading to the hardening of the paint coat while the solvent evaporates and is released to the atmosphere. Any over-sprayed paint and solvent are essentially wasted due to the non-recyclability. Since most of the liquid solvents are organic compounds, they cause serious environmental problems. Legislation and environmental concerns have led to the development of a new alternative coating procedure, which is called the xe2x80x9cpowder coating processxe2x80x9d.
In contrast to the traditional wet coating techniques, the powder coating process involves directly applying a powder paint onto the surface of the part being painted using a carrier gas where the powder is xe2x80x9cheldxe2x80x9d by electrostatic forces. The parts are then put through a curing oven where the powder paint melts and hardens through a series of chemical reactions. Most of the over-sprayed powder paint is recycled. Therefore powder coating is an environmentally friendly technology because it eliminates any organic or inorganic solvents and makes it possible to reuse the over-sprayed paint.
More particularly, a typical powder coating production line consists of a washer, a pre-dryer, a paint booth, a curing oven and a loading/downloading section, as shown in FIG. 1. Parts loaded on the conveyer are first washed to remove dirt, soil and oil. The parts are then dried in a pre-dryer to remove the residual water after which they are ready for powder coating. A paint powder stored in a powder hopper is fluidized by a gas (normally air) and pneumatically transported to a spray gun (either a corona or tribo gun) where it is sprayed onto the surface of the part. Due to the fact that the powder is electrically charged before it reaches the part, the powder will be attracted to the surfaces of the parts that are electrically grounded. When the desired thickness of powder layer has been deposited, the parts are transferred to the curing oven where the paint is melted and hardened. This process has been widely used and it is known that only relatively coarse size powders (normally with average sizes larger than 30-35 microns) can be applied with the existing process and facilities.
Attempts to date to use finer powders have deleteriously resulted in non-smooth powder fluidization, uneven powder transportation and spray (e.g., powder puffing at the gun tip), non-uniform coating surface as well as other undesirable situations. The difficulties with fluidization and pneumatic transportation of fine powders are due to their poor flowability. Generally, when the average size of powder particles is smaller than 20-30 microns, the distances between particles become so small that van der Waals and other interaction forces between particles dominate and the powder becomes very cohesive. Large numbers of particles cling to each other resulting in the formation of agglomerates and/or clumps and cakes. As a result it becomes difficult to fluidize the powder when air passes through the powder bed (or powder-fluidizing hopper). For example, a non-uniform and/or non-stable fluidization, or channeling, will occur in the powder-fluidizing hopper. This leads to a non-stable powder supply (or in many cases, no viable powder supply) to the powder dispensing system such as a spray gun.
In addition, when fine powder is being transported from the powder hopper to the spray gun through the transportation hose, the fine powder tends to stick to and accumulates on the inside of the hose and gun, causing puffing and choking. At bends or other locations where the direction of powder flow is changed, accumulations of hardened fine powder are often present. In the powder coating industry, this is referred to as impact fusion. The paint powder that is sprayed onto part surfaces also exhibits a chunky appearance rather than giving smooth coverage. This will lead to a bumpy finish after curing of the powder. The combination of the poor fluidization and transportation as well as the irregular finish makes the application of fine paint powders impractical.
Thus, the current technology for application of powder coatings does not provide as high a finish quality as xe2x80x9cwet coatingsxe2x80x9d, hindering the further growth of the application of this technology. As mentioned by Bok et al in U.S. Pat. No. 5,171,613, powder coatings are generally characterized as having poor film uniformity, poor distinctness of image and either poor gloss or good gloss with a concomitant heavy orange peel look. Also, excessive film thickness is required to obtain even such limited performance properties. On the other hand, it is difficult to obtain thin films due to the large particle size. Currently, many important coatings, such as color coats and clear coats on car/truck bodies, are still applied by wet coating, due to the quality problems and excessive thickness associated with powder coatings.
The lower quality surface finish of powder coating (xe2x80x9corange peelxe2x80x9d imperfections etc.) and unnecessary excessive thickness, normally 50 microns and upwards, are mainly caused by the larger average particle size than 30-35 microns currently used in the powder coating industry. It is well understood that fine powders with average particle size of less than 20-30 microns can greatly improve the quality of powder coating finishes, making them comparable with wet coating finishes. At the same time, the said fine powders can also make it possible to apply thin film coatings of 10 to 25 microns or even less. Yet the main difficulty causing the application of fine powders to be impractical is the inability to smoothly fluidize and pneumatically transport them. Since such fine powders normally fall into Group C of the Geldart""s classification, they tend to agglomerate badly, making their handling extremely difficult, if not impossible. Therefore, solutions to these problems will break the barrier to the applications of fine paint powder and open up a very promising market for the powder coating industry.
U.S. Pat. No. 5,635,548 teaches that the flowability of fine paint powders may be increased by dry-blending at least two different additives from the following list of inorganic additives: alumina, aluminum hydroxide, calcium oxide, silica, zinc oxide, zirconia, molybdenum trioxide, ceric oxide, tungsten trioxide and aluminum silicate. The most effective additives, according to the inventors, are alumina, aluminum hydroxide, aluminum silicate and silica. The additives have been identified empirically by experimentation without a theoretical framework. Most of the above additives are ceramic or mineral in nature.
U.S. Pat. No. 5,470,893 discloses powder coating compositions with different additives provided to serve various purposes, for example a granulating agent or an additive which adds a metallic luster appearance to the resulting coating.
In order to resolve these problems that are encountered in the powder coating industry as well as in other industries, a method of improving the flowability or flow characteristics of fine powders is needed to facilitate large scale usage of the fine powders in such areas as coatings and the like. Thus it would be very advantageous to provide a method of increasing the flowability or transportability of ultrafine powders using more effective and a much broader selection of measures than is currently available.
The present invention is based on a mechanism, recently discovered by the inventors, for how an additive of a powder comprising particles of smaller size than the average size of the fine powder can increase flowability of the fine powder. Specifically, the inventors have discovered that increased flowability is observed when an additive in powder form, which is to be added to the powder whose flowability is to be increased, has both a smaller size than the fine powder and to a mean apparent particle density less than the mean apparent particle density of the fine powder. The particles of the additive act to separate the fine powder particles and reduce the van der Waals and other possible interparticle forces allowing the flowability of the fine powder to be increased. In addition, the particles of the additive also tend to cling to the surface of the fine powder particles and may serve as xe2x80x9crolling wheelsxe2x80x9d when the fine powder is being handled.
In one aspect the present invention provides a method of increasing flowability of a powder, comprising:
dry blending said powder with an effective amount of a fluidization additive to produce a dry blended mixture, said powder comprising first particles having a first volume-mean equivalent particle size and a first mean apparent particle density, said fluidization additive comprising a second powder with particles of said second powder having a selected second volume-mean equivalent particle size less than said first volume-mean equivalent particle size and having a selected second mean apparent particle density less than said first mean apparent particle density.
In another aspect of the invention there is provided a powder composition, comprising:
a first powder comprising particles having a first volume-mean equivalent particle size and a first mean apparent particle density; and
a second powder comprising second particles having a selected second volume-mean equivalent particle size less than said first volume-mean equivalent particle size and having a selected second mean apparent particle density less than said first mean apparent particle density.
In another aspect of the invention there is provided an article coated with a paint coating derived from a powder composition, the powder composition comprising:
a paint powder comprising first particles having a first volume-mean equivalent particle size and a first mean apparent particle density; and
a second powder comprising second particles having a selected second volume-mean equivalent particle size less than said first volume-mean equivalent particle size and having a selected second mean apparent particle density less than said first mean apparent particle density.